This invention relates generally to sensing while drilling, and more particularly to measuring pressures in drilling mud.
U.S. Pat. No. 2,290,408 of Wilbur J. Crites has a detailed description of problems which may occur as well drill bit chews its way through various geological formations. One problem is the intrusion into the well bore of gas or liquid which lowers the density of the drilling mud and is therefore a precursor of a potential blowout. A second problem occurs when the mud fails to cake properly on the wall of the well bore permitting the intrusion of sand and possibly resulting in an accumulation of sand and cuttings sufficient to bind the drill string. Sand intrusion usually results in increased drilling mud density.
The Crites patent proposed to measure the difference in volume between the drilling mud which was pumped down the hole and that which returned as an indicator of potential trouble. Without questioning the effectiveness of Crites' approach in 1941, it is evidently inadequate when well bore depths of 10 and 20 thousand feet are involved because of the time which will have elapsed when the drilling mud reaches the surface.
In addition to the foregoing problems, efficient operation of the drill bit requires flushing of the media broken up by the bit off of the bit so as not to reduce its drilling capability. This is accomplished by having the drilling mud passing through the drill string at a sufficiently high pressure so that it passes through the nozzles in the bit with suitable velocity and force. For different bits, different pressures may be desired. To ensure that the drilling mud is pumped down the drill string at the proper pressure it is necessary to know the actual pressure near the drill bit both inside the drill string and outside.